Radiation detector including means for indicating satisfactory operation

ABSTRACT

A radiation detector, comprising a sensitive volume filled with a counter gas; an anode and a cathode each in communication with the counter gas; a voltage supply for maintaining a potential difference between the anode and the cathode, said potential difference being less than required to cause gas discharge in the counter gas. The radiation detector further comprises a photoemissive material in communication with the sensitive volume. The photoemissive material may be provided as a coating on the cathode. The radiation detector may further comprise a controllable light source for supplying photons of a known wavelength to the photoemissive material. Electrons may be emitted by the photoemissive material in response to the provision of said photons, said electrons causing avalanche breakdown of the counter gas, indicating satisfactory operation of the radiation detector.

A Geiger-Müller (GM) tube is a gas-filled radiation detector. Itcommonly takes the form of a cylindrical outer shell (cathode) and thesealed gas-filled space with a thin central wire (the anode) held at ˜1kV positive voltage with respect to the cathode. The fill gas isgenerally argon at a pressure of less than 10⁴ Pa plus a small quantityof a quenching vapour.

If a gamma-ray interacts with the GM tube (primarily with the wall byeither the Photoelectric Effect or Compton scattering) it will producean energetic electron that may pass through the interior of the tube.

Ionisation along the path of the primary electron results in low energyelectrons that will be accelerated towards the centre wire by the strongelectric field. Collisions with the fill gas produce excited states(˜11.6 eV) that decay with the emission of a UV photon and electron-ionpairs (˜26.4 eV for argon). The new electrons, plus the original, areaccelerated to produce a cascade of ionisation called “gasmultiplication” or a Townsend avalanche. The multiplication factor forone avalanche is typically 10⁶ to 10⁸. Photons emitted can eitherdirectly ionise gas molecules or strike the cathode wall, liberatingadditional electrons that quickly produce additional avalanches at sitesremoved from the original. Thus a dense sheath of ionisation propagatesalong the central wire in both directions, away from the region ofinitial excitation, producing what is termed a Geiger-Müller discharge.

The intense electric field near the, anode collects the electrons to theanode and repels the positive ions. Electron mobility is ˜10⁴ m/s or 10⁴times higher than that for positive ions. Electrons are collected withina few μs, while the sheath of massive positive ions (space charge)surrounding the centre wire are accelerated much more slowly (ms)outward towards the cathode.

The temporary presence of a positive space charge surrounding thecentral anode terminates production of additional avalanches by reducingthe field gradient near the centre wire below the avalanche threshold.If ions reach the cathode with sufficient energy they can liberate newelectrons, starting the process all over again, producing an endlesscontinuous discharge that would render the detector useless. An earlymethod for preventing this used external circuitry to “quench” the tube,but the introduction of organic or halogen vapours is now preferred. Thecomplex molecule of the quenching vapour is selected to have a lowerionisation potential (<10 eV) than that of the fill gas (26.4 eV). Uponcollision with a vapour molecule the fill gas ion gives up ˜10 eV to thequench vapour molecule which then quickly dissociates rather than losingits energy by radiative emission. The remainder of the partiallyneutralised vapour-atom energy (˜4 eV) produces a UV photon that isstrongly absorbed by the molecules and prevented from reaching thecathode. Any quench vapour that might be accelerated and impact thecathode dissociates on contact. Organic quench vapours, such asalcohols, are permanently altered by this process, limiting tube life to˜10⁹ counts. Halogen quench vapours dissociate in a reversible mannerlater recombining for an essentially infinite life.

Geiger-Müller tubes are a simple, robust and well-established technologyfor the measurement of ionising radiation, insensitive to other effectsand capable of use in many extreme environments. However, as with anyinstrument, it is possible for the detector to fail and cease respondingto ionising radiation. This can be a serious deficiency, particularly ifthe instrument is used in a safety system to give warning of highradiation levels in an area of typically low radiation field. Toovercome this deficiency a small radioactive ‘keep-alive’ source,typically a small β-source of localised emission, is often fitted in oradjacent to the GM tube, to provide a known small background of ionisingradiation. The GM tube is then expected to provide a measurement signalindicating the presence of ionising radiation of at least the levelprovided by the “keep-alive” source. Failure of the detector thenbecomes apparent after a time dependent on the magnitude of the fieldand the sensitivity of the detector, since the GM tube will produce anoutput indicating a level of ionising radiation less than that providedby the “keep-alive” source.

This approach has several drawbacks. Firstly, recent developments in thesensitivity of GM tubes mean that the minimum external radiation fieldthat can be detected is now comparable with that due to the ‘keep alive’source. Accordingly, the measurement of low levels of external ionisingradiation is hampered by the presence of the “keep-alive” source, whosesignal tends to swamp any external measurements of very low value. Inaddition, the inclusion of a radioactive source in an instrument isconsidered undesirable for reasons of long term safety of personneloperating the instrument. The provision of radioactive sources withininstruments also poses problems for end of life disposal, given theever-tightening regulatory framework regulating the disposal ofradioactive materials. The use of radioactive sources is generally alsoto be discouraged due to the potential for environmental contamination.

Other ionising radiation detectors based on, for example, scintillatorsand photodetectors may be less rugged and more sensitive to externaleffects, such as temperature, but their continued operation may beconfirmed by testing with an optical pulser. This is not possible withknown GM tubes.

The present invention aims to alleviate at least some of the problems ofthe known systems.

Accordingly, the present invention provides a radiation detector,comprising a sensitive volume filled with a counter gas; an anode and acathode each in communication with the counter gas; a voltage supply formaintaining a potential difference between the anode and the cathode,said potential difference being less than required to cause gasdischarge in the counter gas. The radiation detector further comprises aphotoemissive material in communication with the sensitive volume.

The photoemissive material may be provided as a coating on the cathode.

The present invention also provides a radiation detector, comprising aGeiger-Müller tube having a photoemissive coating on its cathode.

The radiation detector may further comprise a controllable light sourcefor supplying photons of a known wavelength to the photoemissivematerial, whereby electrons are emitted by the photoemissive material inresponse to the provision of said photons, said electrons causingavalanche breakdown of the counter gas, indicating satisfactoryoperation of the radiation detector.

The light source may comprise one of: a light-emitting diode and anincandescent light bulb. The light source may emit light of visiblewavelengths. Photons from the light source may be provided to thephotoemissive material via an optical fibre. The light source may beplaced within the sensitive volume.

In certain embodiments of the invention, the cathode is in the form of ahollow cylinder, and the anode is in the form of a conductorsubstantially aligned with the axis of the cylinder.

The photoemissive material may comprise at least one rare-earth oxide.

The above, and further, objects, characteristics and advantages of thepresent invention will become more apparent with reference to thefollowing description of certain embodiments, given by way of examplesonly, in conjunction with the accompanying drawings.

FIGS. 1-4 each illustrate a radiation detector according to a respectiveembodiment of the invention.

The present invention provides a modification to radiation detectorssuch as Geiger-Müller (“GM”) tubes. The modification enables a radiationdetector to be tested in-situ without the need for a radioactive source.

According to an aspect of the present invention, the radiation detectoris made sensitive to non-ionising radiation, typically light in a rangeof wavelengths including infra-red, visible and ultra-violet, to enablean optical test pulse to trigger breakdown within the radiation detectorand confirm continued satisfactory operation.

The present invention provides a photo-emissive material within theradiation detector, which, when exposed to light, emits electrons, whichwill then trigger avalanche breakdown of the radiation detector,confirming its continued satisfactory operation.

Suitable photo-emissive materials are typically rare-earth oxides ormixtures of rare-earth oxides, such as are currently used for example ascathode coatings in the construction of photo-multiplier tubes toproduce their sensitivity to incident light.

When such a material is introduced into the sensitive volume of a GMtube and exposed to an optical pulse of an appropriate wavelength, thephotoelectrons produced will trigger avalanche breakdown and produce anelectrical output pulse, confirming continued satisfactory performanceof the detector. The photo-emissive material could be coated onto thecathode of the radiation detector for optimal efficiency in generating abreakdown.

A control circuit is provided, which supplies activation energy, such asa voltage pulse, to the light source at predetermined intervals. Thecontrol circuit will then monitor the output of the radiation detectorfor a measurement pulse corresponding to the activation of the lightsource. If a corresponding measurement pulse is provided, then theradiation detector is confirmed as operating satisfactorily. If nomeasurement pulse is provided, then the radiation detector has a fault.Of course, the fault may lie within the control circuit or the lightsource, meaning that no photons are supplied to the photoemissivematerial. However, the radiation detector itself may be at fault. Theradiation detector should then be removed from use immediately andreplaced or serviced.

The optical pulse can be generated in several ways. The light source maybe, for example, a light emitting diode (LED) or an incandescent sourcesuch as a light bulb. The light source may be integral to the radiationdetector, for example placed within the sensitive volume in which caseprovision must be made for supplying activation energy to the lightsource. Alternatively, the light source may be arranged to providephotons through a transparent or translucent window in the radiationdetector. The light source may be placed distant from the radiationdetector, with geometrical optics or an optical fibre provided to carryphotons from the light source to the photo-emissive material. Theradiation detector may be fitted with an optical fibre or window tocouple to an external light source.

FIG. 1 illustrates a first embodiment of the present invention, in theform of a GM tube 10. A hollow cylindrical cathode 20 encloses anddefines a sensitive volume 22. An anode 24 in the form of a wire or baris provided along the length of the cathode, substantially along theaxis of the cylinder. A gas tight enclosure (not illustrated) isprovided, enclosing the anode, the cathode and the sensitive volume. Avoltage source 25 maintains a potential difference of typically severalhundred volts between the anode and the cathode. A resistance 26 isprovided, to convert the current pulses caused by discharge in the tube10 into voltage pulses. The voltage pulses may be capacitively coupledto a loudspeaker 28 and/or a counter 30 to provide an audible and/orvisual indication of the discharge within the tube 10. The system ofFIG. 1 described thus far, in this paragraph, is conventional.

According to certain aspects of the present invention, a photoemissivematerial is provided within the sensitive volume 22. In the particularembodiment shown in FIG. 1, the photoemissive material is provided as aphotoemissive coating 32 over the entire inner surface of the cathode20. According to another aspect of the present invention, a light source34 is provided, in a location selected such that photons emitted by thelight source 34 may reach the photoemissive coating 32. The light source34 may be within, or outside of the cathode 22. The light source 34 maybe within, or outside, the gas tight enclosure (not shown). Geometricaloptics or an optical fibre may be provided to carry photons from thelight source 34 to the photo-emissive material 32. Selection of thelocation of the light source may depend on may factors. For example,having an external light source will ease the task of replacingincandescent bulbs. Placing an LED light source inside the cathode willresult in a smaller overall device. Placing the light source in alocation where it may be directly observed by a user will provide thereassurance that the light source is working, and so that the radiatordetector is being correctly tested.

The light source 34 may emit light in the infra-red, visible orultra-violet wavelength ranges. The light source must be selected suchthat the photons it emits are of suitable wavelength the release photonsfrom the photoemissive material 32. The photoemissive material 32 may bemade up of one or more rare earth oxides, for example rubidium oxide,caesium oxide, thorium oxide or cerium oxide.

Control circuitry 36 supplies activation energy, such as a voltagepulse, to the light source 34. If the light source and the radiationdetector are working correctly, a corresponding return voltage pulsewill be provided by the GM tube 10, and detected by the loudspeaker 28and/or counter 30. The return voltage pulse is also provided to thecontrol circuitry 36. If no return voltage pulse is detected by thecontrol circuitry, this indicates a malfunction and the controlcircuitry may cause an alarm signal to the operator. Since the returnvoltage pulse provided in response to the activation of the light sourcedoes not indicate a real radiation detection, the control circuitry maybe arranged 38 to deduct the corresponding value from the counter 30.

The control circuitry repeats this testing operation at predeterminedintervals. The interval may be selected in accordance with theenvironments of the radiation detector. In some environments, a testingrate of once every ten minutes or once per hour may be sufficient. Inother applications, it may be appropriate to repeat the test everyminute, or less. Each test cycle may comprise a single activation of thelight source, or may involve a number of repeated activations, forexample, six activations at one second intervals every ten minutes. Thismay provide for some error filtering: if five of the six measurementsindicate that the radiation detector is functioning normally, then oneabnormal result may perhaps be ignored.

FIG. 2 shows a second embodiment of the present invention. In thisembodiment, the photoemissive coating 32 is provided only on a portionof the inner surface of the cathode 20. The portion coated in thephotoemissive material must lie in a line of sight from the lightsource, via any optical fibre or geometrical optics which may beprovided. This embodiment may reduce the cost of the photoemissivematerial used, and may alleviate any adverse effects of thephotoemissive coating on the operation of the radiation detector.

FIG. 3 shows a third embodiment of the present invention. In thisembodiment, the light source, probably an LED in this case, is placedwithin the sensitive volume 22. A relatively small area of the cathodeis coated with photoemissive material 32. This small area is placed in alocation which will receive photons emitted by the light source 34. Thisembodiment will provide a smaller overall device, and will furtheralleviate any adverse effects of the photoemissive coating on theoperation of the radiation detector.

FIG. 4 illustrates a further embodiment of the present invention,wherein the photoemissive material 32 is provided upon a carrier 44mounted generally within the radiation detector. Carrier 44 may be anelectrically insulating material such as mica or polyethylene, mountedon the anode 44. The carrier and particularly the coating 32 must beplaced in a location where it will receive photons from the light source34. This embodiment illustrates that it is not necessary for thephotoemissive material to be applied to the cathode of the radiationdetector.

The present invention accordingly provides a radiation detector with aself-testing and monitoring function, which avoids the need for aradioactive “keep-alive” source to be provided. This provides thefurther advantages of enabling the radiation detector to detect lowerlevels of external radiation, while avoiding the problem of long termexposure of operators to potentially harmful radiation, reduces the riskof environmental contamination and simplifies end of life disposal ofthe radiation detector.

While the present invention has been described with reference to alimited number of particular embodiments, the invention is not solimited. The present invention is limited only as recited in theappended claims.

1. A radiation detector for detecting ionizing radiation having awavelength within a first range, said radiation detector comprising:detecting means responsive to said ionizing radiation incident thereon,for generating an output signal indicative of said ionizing radiation;and self-testing means for determining that said detecting meanscontinues to operate properly to detect ionizing radiation, said selftesting means comprising, a photoemissive material in communication withthe detecting means; a controllable light source for supplyingnonionizing radiation of a known wavelength to the photoemissivematerial, which known wavelength is within a second range that differsfrom said first range, whereby ionizing radiation is emitted by thephotoemissive material in response to the provision of said nonionizingradiation, said ionizing radiation impinging on the detecting means; andcontrol means for supplying activation energy to the light source atpredetermined intervals, monitoring the output of the radiation detectorto detect any measurement pulses corresponding to activation of theradiation detector by the activation of the light source, deciding inresponse to such detection that the radiation detector is satisfactorilyresponsive to ionizing radiation, and deciding, in the absence of suchdetection that the radiation detector has a fault.
 2. A radiationdetector for detecting incident ionizing radiation, said radiationdetector comprising detecting means responsive to ionizing radiationincident thereon, for generating an output signal indicative of saidincident ionizing radiation, said detecting means comprising a sensitivevolume filled with a counter gas; an anode and a cathode each incommunication with the counter gas; and a voltage supply for maintaininga potential difference between the anode and the cathode, said potentialdifference being less than required to cause gas discharge in thecounter gas, such that incident ionizing radiation causes avalanchebreakdown of the counter gas, which breakdown is detected by thedetecting means, wherein the radiation detector further comprises: aphotoemissive material in communication with the sensitive volume of thedetecting means; a controllable light source for supplying photons ofnon-ionizing radiation of a known wavelength to the photoemissivematerial, whereby electrons are emitted by the photoemissive material inresponse to the provision of said photons, said electrons causingavalanche breakdown of the counter gas of the detecting means, whichbreakdown is detected by the detecting means, indicating satisfactoryoperation of the radiation detector; and control means for i) supplyingactivation energy to the light source at predetermined intervals, ii)monitoring the output of the radiation detector to detect anymeasurement pulses corresponding to activation of the radiation detectorby activation of the light source, iii) deciding, in response to suchdetection, that the radiation detector is satisfactorily responsive toionizing radiation; and iv) deciding, in the absence of such detection,that the radiation detector has a fault.
 3. The radiation detectoraccording to claim 2, further arranged to generate an alarm in responseto deciding that the radiation detector has a fault.
 4. The radiationdetector according to claim 2, further arranged to deduct themeasurement pulses corresponding to activation of the radiation detectorby the activation of the light source from the output of the radiationdetector.
 5. The radiation detector according to claim 2, furtherarranged such that, at each interval, repeated activations of the lightsource occur, and the decision on the state of the radiation detector isdecided with error filtering.
 6. The radiation detector according toclaim 2, wherein the photoemissive material is provided as a coating onthe cathode.
 7. The radiation detector according to claim 2, wherein theradiation detector comprises a Geiger-Müller tube having a photoemissivecoating on its cathode.
 8. The radiation detector according to claim 2,wherein the cathode is in the form of a hollow cylinder, and the anodeis in the form of a conductor substantially aligned with the axis of thecylinder.
 9. The radiation detector according to claim 2, wherein thelight source comprises one of: a light-emitting diode and anincandescent light bulb.
 10. The radiation detector according to claim2, wherein the light source emits light of visible wavelengths.
 11. Theradiation detector according to claim 2, wherein photons from the lightsource are provided to the photoemissive material via an optical fiber.12. The radiation detector according to claim 2, wherein the lightsource is placed within the sensitive volume.
 13. The radiation detectoraccording to claim 2, wherein the photoemissive material comprises atleast one rare-earth oxide.
 14. A method for testing operational statusof a detector for detecting incident ionizing radiation that has awavelength within a first range, said method comprising: providing aphotoemissive material in communication with a detecting unit thatoutputs a signal in response to said ionizing radiation incidentthereon; irradiating said photoemissive material with nonionizingradiation having a wavelength which is within a second range thatdiffers from said first range, and which causes said photoemissivematerial to irradiate said detecting unit with ionizing radiation havinga wavelength that is within said first range; determining that thedetector is properly responsive to ionizing radiation if a signal isoutput in response to said light pulses; and determining that thedetector is not properly responsive to ionizing radiation if no suchsignal is output.
 15. A method for detecting external ionizingradiation, comprising: providing a detector that emits signals inresponse to ionizing radiation incident thereon, which ionizingradiation has a wavelength within a first range; and periodicallychecking responsiveness of said detector to said ionizing radiation, byproviding a photoemissive material in communication with said detector:irradiating said photoemissive material with nonionizing radiationhaving a wavelength which is within a second range that differs fromsaid first range, and which causes said photoemissive material toirradiate said detector with ionizing radiation having a wavelength thatis within said first range; determining that the detector is properlyresponsive to ionizing radiation if a signal is output in response tosaid light pulses; and determining that the detector is not properlyresponsive to ionizing radiation if no such signal is output.